This invention relates to optical or photonic components, more particularly to optoelectronic devices formed of polymers.
Integrated optical devices (i.e., waveguides, switches, interconnects, and the like) are known which are constructed of polymer materials having a glass transition temperature (Tg) much higher than the operating temperature range of the device. The glass transition temperature is a range of temperatures over which significant local motion of the polymer backbone occurs. The Tg is usually defined as cooperative motion of about 10 backbone units, or a viscosity of 10.sup.14 poise, or a second order phase transition in heat capacity. The temperature at which the change in slope occurs in the rate of change of volume with temperature is considered the glass transition temperature (Tg), or softening point. For a detailed description of viscoelasticity and characteristics illustrated by FIGS. 1 and 2, see G. B. McKenna, chapter 10, Comprehensive Polymer Science, Volume 2, Edited by C. Booth and C. Price, Permagon Press, Oxford (1989).
Crosslinked materials manifest a glass transition when the molecular weight between crosslinks is significant enough to allow cooperative motion of the backbone units. Thus, a lightly crosslinked material will show a glass transition; while a highly crosslinked one may not.
Below Tg, polymeric material is prevented from reaching equilibrium because of the limited amount of segmental motion. Thermodynamic (entropic) effects still drive change towards equilibrium, but if the temperature is far enough below Tg, those changes will occur at such a slow rate that it does not appear experimentally during the time scale of interest (in this case the time scale of observation).
There are several reasons why materials having a high Tg have been chosen in the past, including compatibility with electronics processing and packaging, maintenance of orientation of chromophores incorporated within the material, and environmental robustness and performance stability. The use of high Tg materials (materials with a Tg higher than the operating temperature of the materials in a packaged device) ensures device operation in a region in which the local motion of the polymer segments is significantly restricted, and that the material operates in a glassy state. It was assumed in the earliest development of polymer films for optoelectronic devices that use of high Tg materials was a requirement. "For example, many of the first research EO polymers, whether guest-host or side chain, are based on thermo-plastic acrylate chemistry and exhibit glass transition temperatures.about.100-150.degree. C. This low Tg results in high polymer chain diffusion rates and a variation of at least 10% in the optical properties of the poled state over 5 years of operation at ambient temperature. This rapid change is the natural consequence of the dynamic processes by which glassy polymers, operating close to Tg, undergo physical aging and relaxation to reduce stress and minimize free volume. When higher operating temperatures are considered (125.degree. C.), the stability of the optical properties becomes even worse." (extracted from the review paper by R. Lytel et al., in Polymers for Lightwave and Integrated Optics, L. A. Hornak, ed., Marcel Dekker 1992 pp. 460).
Higher glass transition materials developed for integrated optoelectronics include polyimide materials (glass transitions ranging from about 250.degree. C. to well over 350.degree. C.) developed by Hoechst, DuPont, Amoco, and others, and polyquinolines (Tgs greater than 250.degree. C.) developed by Hitachi Chemical. The researchers were guided by the presumption that "The first priority for such waveguides should be high thermal stability to provide compatibility with high-performance electronics device fabrication. The fluorinated polyimides have a high glass transition temperature above 335.degree. C., and are thermally stable against the temperatures in IC fabrication processes involving soldering (.about.270.degree. C.)." (T. Matsuura et al., Elect. Lett. 29 2107-2108 (1993)).
The requirements for polymers used in thermo-optic switches are reported by R. Moosburger et al. (Proc. 21st Eur. Conf. On Opt. Comm. (ECOC95-Brussels) p. 1063-1066). "Low loss switches at a wavelength of 1.3 .mu.m were fabricated with the commercially available and high temperature stable (Tg&gt;350.degree. C.) polymer CYCLOTENE.TM. . . . CYCLOTENE.TM. was chosen due to its low intrinsic optical loss, thermal stability in excess of 350.degree. C., low moisture uptake and excellent planarisation properties."
The requirements for polymers for polymer passive optical interconnects are reported by DuPont for their Polyguide.TM. material system in R. T. Chen et al., SPIE Vol. 3005 (1997) p. 238-251, "High Tg and low coefficient of thermal expansion (CTE) polymers provide thermal-mechanical and environmental robustness and performance stability through their complete domination of the Polyguide.TM. packaged structure properties." DuPont uses cellulose acetate butyrate (CAB) materials as described in U.S. Pat. Nos. 5,292,620 and 5,098,804.
In addition to the acrylate, polyimide, polyquinoline, benzocyclobutene and CAB materials systems mentioned above, other materials systems that have been used to make integrated optical devices include cardo-polymers (C. Wu et al., in Polymer for Second-Order Nonlinear Optics, ACS Symposium Series 601, pp. 356-367, 1995), epoxy composites, (C. Olsen, et al., IEEE Phot. Tech. Lett. 4, pp. 145-148, 1992), polyalkylsilyne and polysilyne (T. Weidman et al., in Polymers for Lightwave and Integrated Optics, Op. Cit. pp. 195-205, 1992), polycarbonate and polystyrene (T. Kaino, in Polymers for Lightwave and Integrated Optics, Op. Cit., pp. 1-38, 1992), polyester (A. Nahata et al., Appl. Phys. Lett. 64, 3371, 1994), polysiloxane (M. Usui et al., J. Lightwave Technol. 14 2338, 1996), and silicone (T. Watanabe et al. J. Lightwave Technol. 16 1049-1055, 1998). Poly methyl methacrylate, polystyrene, and polycarbonate have also been used for polymer optical fibers (POFs). Polycarbonate is used as compact disc substrates, and is used in plastic eyeglass lenses, hard contact lenses, and related applications. Silicones are used in flexible contact lenses.
Several researchers have designed optical switching devices using thermal effects in polymers. In addition to the research work of R. Moosburger, Op. Cit., one group has been trying to commercialize thermo-optic switches using a digital optical waveguide switch configuration (G. R. Mohlmann et al., SPIE Vol. 1560 Nonlinear Optical Properties of Organic Materials IV, pp. 426-433, 1991). In this work, a resistive heating element is deposited on a high glass transition temperature thermo-optic polymer stack that contains a waveguide y-branch splitter. Activation of a heater electrode produces a decrease in the refractive index under the activated electrode and results in light switching into the waveguide branch that is not activated.
In work with polymers for thermo-optic integrated optical devices leading to the present invention, it has been observed that there are nonlinear responses due to the viscoelastic behavior of the materials. After repetitive switching of a thermo-optic device, for instance, the polymers begin to exhibit a local change in index of refraction where they were heated, disturbing the "off" state of the switch and its time response. The viscoelastic properties of a polymer determine the mechanical character of the material response to applied heat or other perturbation. These properties control the rate at which applied changes (such as heat, stress, acoustic excitation, etc.) produce time-dependent responses in the material properties (such as evolution of the index of refraction, mechanical strain, etc.). Any truly elastic contribution generally is linear and disappears after the applied change is removed. However, time-dependent elements of the material response are retained within the material after the removal of the applied change and may require minutes to eons for restoration. If the material response results in a degradation of the operating characteristics of a device, that degradation may accumulate over time and result in failure of the device to meet performance specifications.
For optical devices used in communications, such behavior is undesirable because it can degrade the insertion loss, crosstalk immunity and other performance measures that are critical to the bit error rate of the system. Any such factor that changes with time is a problem for telecommunications applications, where reliability and reproducibility are essential, but where a broad range of environmental conditions may be encountered during a service lifetime. To enable effective thermo-optic switching devices, materials should not exhibit any such slow changes in optical properties.